1. Field of the Invention
The present invention relates to a liquid injection apparatus and, more particularly, to a liquid injection apparatus having a residual ink quantity detecting means for detecting the presence/absence of ink from a change in pressure in an ink supply system and to a residual ink quantity detecting apparatus for detecting a residual quantity of ink in an ink supply source such as an ink tank.
2. Related Background Art
An ink-jet printer, which ejects ink from a small nozzle to record an image or character on a recording medium has become popular. In a printer of this type, no ink ribbon is used, and ink liquid stored in an ink tank is ejected as small ink droplets to a recording medium such as a paper sheet through an injection mechanism of a printing head, which is connected to the ink tank through a tube, thereby recording an image and the like thereon. In the ink-jet printer of this type, a mechanism for detecting a residual quantity of ink in the ink tank is normally arranged in order to prevent print errors due to ink shortage. When the residual quantity of ink becomes insufficient, an alarm sound is produced, thereby urging an operator to refill ink or to exchange an ink tank cartridge.
In some ink-jet printers in which an ink tank as an ink supply source fixed to the main body of the ink-jet printer or a recording head is mounted on a carriage, a conventional residual ink quantity detecting apparatus is arranged on a sub-ink tank mounted on the carriage. In the conventional residual ink quantity detecting apparatus, a float which includes a magnet and moves in accordance with a change in liquid level is detected by a lead switch, or a light emitting means and a light receiving means are arranged, so that when the ink level is decreased below a predetermined value, light emitted from the light emitting means reaches the light receiving means without being sealed by the ink, thereby detecting a residual quantity of ink.
However, in the conventional residual ink quantity detecting apparatus in the ink-jet printer, in particular, when it is provided to a fixed ink tank and the ink tank is flat and is constituted by a flexible bag, a change in liquid level upon decrease in quantity of ink is small. Therefore, the residual quantity of ink cannot be accurately and reliably detected.
The conventional residual ink quantity detecting apparatus must be exchanged together with the ink tank when the ink tank is exchanged. Therefore, the ink tank becomes expensive.
When the residual ink quantity detecting apparatus is provided to a sub-ink tank mounted on a carriage, the liquid level in the sub-ink tank fluctuates when the carriage is moved in a predetermined direction. Therefore, an erroneous operation may occur upon residual ink quantity detection. In addition, an electrical connecting means necessary for the residual ink quantity detecting apparatus is moved together with the carriage. Therefore, the connecting portion may present a reliability problem.
Thus, a residual ink quantity detecting apparatus which is arranged midway along an ink supply path between a recording head and an ink tank has been proposed.
FIG. 1 shows an open-air type conventional residual ink quantity detecting apparatus, as the residual ink quantity detecting apparatus of the above type. An open-air type manometer 200 is arranged midway along an ink supply path. A pair of ink presence/absence detectors 200a and 200b such as electrodes detect a decrease in ink level I.
FIG. 2 shows a closed type conventional residual ink quantity detecting apparatus. In FIG. 2, a flexible film (diaphragm) 301 constitutes part of an ink supply path. An electrode 300 is provided to the diaphragm 301. A stationary electrode 302 is brought into contact with the electrode 300 when the diaphragm 301 is moved downward in FIG. 2 upon increase in negative pressure in the ink supply path. A spring member 303 biases the diaphragm 301 upward in FIG. 2. Note that as the closed type detector, a pair of fixed photosensors are arranged, and a light shielding plate is provided to a diaphragm to be capable of shielding the optical path, thereby detecting its deviation.
However, in the ink-jet printer having the open-air type apparatus described above, an ink tank must be arranged above the level of the ink presence/absence detectors 200a and 200b. Therefore, the total height of the printer must be inevitably increased, and the printer becomes bulky. When a decrease in ink is detected and an old ink tank is exchanged with a new one, a very long period is required for recovering a liquid level I in the manometer 200. In consideration of an inoperative state or transportation of the ink-jet printer, an opening/closing means for preventing an ink leakage from the manometer and evaporation of ink causing an increase in ink viscosity or ink solidification must be provided. Therefore, the printer becomes bulky, and its manufacturing cost is also increased.
In the closed type residual ink quantity detecting apparatus, for example, in the apparatus having the diaphragm which is partially constituted by a flexible member and detects a residual quantity of ink utilizing a deformation caused by a pressure difference between inside and outside the flexible member, as shown in FIG. 2, a detection output varies due to variations in dimension of the flexible member. Thus, the residual quantity of ink cannot be accurately detected. Since a deformation due to pressure is utilized, adjustment in the overall ink supply system, for example, adjustment of a relative level difference between the recording head and the residual ink quantity detecting apparatus, must be performed for individual ink-jet printers. Since an adjusting mechanism is arranged, the overall apparatus becomes large, and its manufacturing cost is increased.
In the ink-jet printer having the closed type residual ink quantity detecting apparatus, when an old ink tank is exchanged with a new one upon detection of decrease in ink, a very long period of time is required for recovering an initial state of the diaphragm. If a spring member having a large spring constant is used to allow quick recovery, a deviation amount of the diaphragm cannot be set to be large, and detection precision is degraded.
In the ink-jet printer having the closed type residual ink quantity detecting apparatus, the diaphragm arranged as part of the ink supply system must be deviated with good response at very low pressure of a several tens of mmH.sub.2 O at which a degradation of print quality occurs. Therefore, the diaphragm is preferably formed of a material such as a low-hardness rubber having a high flexibility. In addition, its film thickness is preferably decreased to 0.1 to 0.3 mm.
For this reason, ink in the ink supply system may be evaporated through the diaphragm 301 or air enters the ink supply system therethrough, thereby: interfering with ink injection. When the print quality is degraded, a high pressure is applied to ink in the head to perform recovery. In this case, since this pressure is also applied to the diaphragm through the ink supply tube, the residual ink quantity detecting apparatus may be broken.
In the residual ink quantity detecting apparatus which is operated upon change in pressure in the ink supply system, the predetermined negative pressure, i.e., an operating pressure for the residual ink quantity detecting apparatus is very low. Therefore, the residual ink quantity detecting apparatus may be erroneously operated when a change in pressure due to a factor other than the negative pressure produced upon decrease in quantity of ink occurs.
It was found that, in a liquid injection recording apparatus shown in FIG. 3, to which the present invention is applied, the residual ink quantity detecting apparatus was erroneously operated at negative pressure produced when ink is drawn from the distal end of a recording head 1 using a pump 9 or upon change in pressure caused by movement of ink when a carriage 2 is moved.
A change in pressure in the ink system upon movement of the carriage-2 is cause during carriage turn in which the moving direction of the carriage 2 is reversed. In the mechanism shown in FIG. 3, a negative pressure is produced in the residual ink quantity detecting apparatus at the right end (right carriage turn) and a positive pressure is produced at the left end (left carriage turn). For this reason, even though the residual quantity of ink is sufficient, the residual ink quantity detecting apparatus produces an output indicating a "small" residual quantity of ink upon use of the pump and right carriage turn. On the other hand, upon left carriage turn, the apparatus produces an output indicating a "sufficient" residual quantity of ink even if the residual quantity of ink is small.